Biomedical Engineering Reference
In-Depth Information
t
1
=
180 ms
v
=
18 m/s
1
2
mv
2
1
2
×
1
×
18
2
translational KE
=
=
=
162
J
Note that this 162
J
is equal to the work done on the baseball prior to release.
total energy
=
PE
(t
1
)
+
translational KE
(t
1
)
=
19
.
6
+
162
=
181
.
6
J
If we ignore air resistance, the total energy remains constant during the
flight of the baseball, such that at
t
2
, when the maximum height is reached,
all the energy is potential energy and KE
=
0. Therefore, PE
(t
2
)
=
181
.
6J.
This means that the baseball reaches a height such that
mgh
2
=
181
.
6 J, and:
181
.
6
1
.
0
h
2
=
9
.
8
=
18
.
5m
×
At
t
3
the baseball strikes the ground, and
h
=
0. Thus, PE
(t
3
)
=
0 and
KE
(t
3
)
=
181
.
6 J. This means that the velocity of the baseball is such that
1
2
mv
2
=
181
.
6J, or:
v
=
19
.
1m/s
This velocity is slightly higher than the release velocity of 18 m/s because
the ball was released from 2 m above ground level.
6.2.1 Energy of a Body Segment and Exchanges of Energy Within the
Segment
Most body segments contain all three energies in various combinations at
any point in time during a given movement. A diver at the top of a dive
has considerable potential energy, and during the dive, converts it to kinetic
energy. Similarly, a boomerang when released has rotational and translational
kinetic energy, and at peak height some of the translational kinetic energy has
been converted to potential energy. At the end of its travel, the boomerang
will have regained most of its translational kinetic energy.
In a multisegment system, such as the human body, the exchange of energy
can be considerably more complex. There can be exchanges within a segment
or between adjacent segments. A good example of energy exchange within
a segment is during normal gait. The upper part of the body [head, arms,
and trunk (HAT)] has two peaks of potential energy each stride — during
midstance of each leg. At this time, HAT has slowed its forward veloc-
ity to a minimum. Then, as the body falls forward to the double-support
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